The https://wikipedia.org portal is the landing page for visitors to Wikipedia when they visit www.wikipedia.org
Unlike the rest of the Wikipedia site, it is not built with MediaWiki but is a static web page.
The code can be found on Gerrit.
For information on the 2016/17 work by the WMF Discovery team to update the portal, see Wikipedia.org Portal Project.
The portal uses Handlebars and underscore. It is built using gulp and tested with CasperJS. Code changes are committed to the repository and released separately via updates by the automated Gerrit account PortalsBuilder.